commit-reach: fix type of `min_commit_date`

commit-reach: fix type of `min_commit_date`

The `can_all_from_reach_with_flag()` function accepts a parameter that
allows callers to cut off traversal at a specified commit date. This
parameter is of type `time_t`, which is a signed type, while we end up
comparing it to a commit's `date` field, which is of the unsigned type
`timestamp_t`.

Fix the parameter to be of type `timestamp_t`. There is only a single
caller in "upload-pack.c" that sets this parameter, and that caller
knows to pass in a `timestamp_t` already.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>